1. Field of the Invention
This invention relates to a communication device and a communication method to be used for packet communications between two networks that conform to respective standards that are different from each other. The present invention also relates to a program to be used for such packet communications.
2. Description of Related Art
Currently, LANs (local area networks) are in operation to a limited extent. With LANs, computer terminals can communicate with each other and transmit data and images. The existing LANs include IEEE 802 networks, or so-called Ethernet (trademark) networks, that conforms to the IEEE (Institute of Electrical and Electronics Engineers) 802 Standard.
With an Ethernet network, communications are controlled according to the TCP/IP (Transport Control Protocol/Internet Protocol). An IP address is assigned to each computer terminal either manually by the network controller or automatically by DHCP (Dynamic Host Configuration Protocol). Data are transmitted and received on the basis of assigned IP addresses. However, a physical address (inherent address), or MAC (media access control) address, that corresponds to the IP address is required to realize a data transmission. The MAC address is the address number assigned to the NIC (network interface card) of the computer terminal, which is the interface equipment of the computer terminal. The computer terminal uses both the IP address and the MAC address for communications.
Communication devices for connecting networks include bridges adapted to connect LAN segments by way of the data link layer, which is the second layer, and routers adapted to connect LANs by way of the network layer, which is the third layer.
If an Ethernet network and a network other than an Ethernet network, e.g., an IEEE 1394 network that conforms to the IEEE 1394 Standard, are involved, the bridge can connect the Ethernet network and the IEEE 1394 network to produce a single network for relaying data.
However, if a computer terminal is connected to the IEEE 1394 network, the computer terminal does not have a MAC address, while the computer terminals on the IEEE 802 network are provided with a MAC address.
To cope with this problem, the bridge holds in advance a number of MAC addresses that can be used when connecting an Ethernet network and an IEEE 1394 network and assigns one of the MAC addresses to the computer terminal on the IEEE 1394 network for relaying data.
On the other band, the router interprets the header of the IP address of the computer terminal by referring to the routing table when connecting the Ethernet network and the IEEE 1394 network for relaying data.
Thus, the bridge needs to hold MAC addresses as many as the number of computer terminals that can be connected to the IEEE 1394 network. However, if the number of computer terminals that are actually connected to the IEEE 1394 network is smaller than the number of MAC addresses the bridge holds, the remaining MAC addresses are held by the bridge for nothing.
On the other hand, when the router connects the Ethernet network and the IEEE 1394 network, an IP subnet has to be assigned to the IEEE 1394 network to waste some of the limited IP address space. Additionally, since the router segments the scope of broadcast or multicast (broadcast to a specific group), there can arise a problem that it is difficult for a computer terminal on the Ethernet network to detect a specific computer terminal on the IEEE 1394 network (device discovery).